System for on-chip temperature measurement in integrated circuits

ABSTRACT

A thermal sensor providing simultaneous measurement of two diodes. A first diode and a second diode are coupled to a first current source and a second current source, respectively. The ratio of the currents provided by the two sources is accurately known. The voltage across each of the two diodes may be coupled to the input of a differential amplifier for determination of temperature. Alternatively, the first diode may be coupled to a first current source by a resistor with a known voltage drop, and the second diode may be coupled to an adjustable second current source. The current in the second diode may be adjusted until the voltage across the second diode is equal to the sum of voltage drop across the first diode and the known voltage drop across the resistor. Under the established conditions, the Diode Equation may be used to calculate a temperature.

RELATED APPLICATIONS

This patent is a continuation of and claims the benefit of the commonlyowned and co-pending patent application Ser. No. 10/961,311 (attorneydocket number TRAN-P378) filed Oct. 7, 2004, entitled “System ForOn-Chip Temperature Measurement In Integrated Circuits” which is acontinuation of patent application Ser. No. 10/411,955 (attorney docketnumber TRAN-P085) filed Apr. 10, 2003, entitled “System For On-ChipTemperature Measurement In Integrated Circuits” which are herebyincorporated by this reference.

FIELD OF THE INVENTION

Embodiments of the present invention relate to temperature measurementof integrated circuits. In particular, embodiments of the presentinvention relate to an on-chip temperature sensor for integratedcircuits.

BACKGROUND ART

Temperature measurement in semiconductor devices such as integratedcircuits on silicon substrates is often done by taking advantage of thefundamental relationship between the saturation current of a p-njunction and its temperature. This relationship is described by theDiode Equation shown below:

I=I _(s)*[exp(qV/nkT)−1]

where,

-   -   I_(s)=saturation current    -   q=electron charge    -   V=p-n junction voltage    -   n=ideality factor (between 1 and 2)    -   k=Boltzmann's constant    -   T=absolute temperature (K)

The ideality factor n is equal to 2 for pure recombination current (lowvoltage, low current density), and equal to 1 for pure diffusion current(higher voltages). When using a p-n junction as a temperature sensor, itis desirable that n be close to 1. However, high current densitiesshould be avoided to minimize ohmic effects due to series resistancesoutside of the p-n junction. Ohmic effects can lead to a deviation fromthe Diode Equation.

FIG. 1 shows a conventional thermal sensor 100. A current source 105with a single diode 110 is used, with sequential measurements beingtaken for current and voltage to obtain two I-V data pairs (I₁, V₁) and(I₂, V₂) for the diode 110. The temperature T is then calculated(neglecting the −1) from the Diode Equation as follows:

T=(q/nk)*(V ₂ −V ₁)/(ln(I ₂ /I ₁))

The (−1) term in the Diode Equation may be ignored since the resultingerror is usually less than 1 part in 100,000 for all current densitiesof interest.

In conventional temperature measurements made using a single diode,there are a number of error sources that reduce the accuracy andreliability of the measurements. Also, the sequential measurementsreduce the frequency with which measurements can be made.

In the measurement of the two voltages, the error associated with eachindividual measurement contributes to the total error for the term(V₂−V₁). Since this term is normally quite small (about one tenth of V₂or V₁), the accuracy of the voltage measurements is critical. Also,voltage measurements usually involve an analog-to-digital conversion,with an associated quantization error that is counted twice.

Another source of error are leakage currents. For example, shuntresistance 120 may produce a deviation from the I-V characteristicexpressed by the Diode Equation. Also, since the measurements aresequential, short term changes in the circuit state can affect themeasurements. As previously described, a series resistance 115 may alsointroduce error.

SUMMARY OF INVENTION

Thus, a need exists for a more accurate temperature sensor forintegrated circuits. There is also a need for a temperature sensor thateliminates the problems associated with sequential electricalmeasurements, as well as providing reduced errors, reduced noise, and anincreased measurement frequency.

Accordingly, embodiments of the present invention provide on-chiptemperature sensing through simultaneous electrical measurement of aplurality of diodes. The simultaneous measurement of more than one diodeeliminates the need for sequential measurements and reduces quantizationerror.

In an embodiment of the present invention, two diodes are each coupledto a controlled current source. The ratio of the currents provided bythe two sources is accurately known. The voltage across each of the twodiodes is coupled to the input of a differential amplifier. The outputof the differential amplifier may be coupled to an analog-to-digitalconverter.

In another embodiment, a first diode is coupled to a first currentsource by a resistor with a known voltage drop, and a second diode iscoupled to an adjustable second current source. The current in thesecond diode is adjusted until the voltage across the second diode isequal to the sum of voltage drop across the first diode and the knownvoltage drop across the resistor. Under the established conditions, theDiode Equation may be used to calculate a temperature.

Although the above embodiments describe the use of two diodes inparallel, three or more diodes may be used in parallel, with or withoutcoupling resistors. The additional measurements may be used to furtherreduce error.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodimentswhich are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention. The drawings referred to in this description should not beunderstood as being drawn to scale except if specifically noted.

Prior Art FIG. 1 shows a thermal sensor with a single diode.

FIG. 2A shows a schematic diagram for a square layout of two diodes inaccordance with an embodiment of the present claimed invention.

FIG. 2B shows a schematic diagram for an approximate circular layout oftwo diodes in accordance with an embodiment of the present claimedinvention.

FIG. 3A shows a dual-diode thermal sensor, in accordance with anembodiment of the present claimed invention.

FIG. 3B shows a dual-diode thermal sensor with a sensing seriesresistor, in accordance with an embodiment of the present claimedinvention.

FIG. 4 shows a current source servo controller, in accordance with anembodiment of the present claimed invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the present invention, a systemfor on-chip temperature measurement in an integrated circuit, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances well known methodsinvolving photolithography, ion implantation, deposition and etch, etc.,and well known circuit components such as current sources andamplifiers, etc., have not been described in detail so as not tounnecessarily obscure aspects of the present invention.

FIG. 2A shows a substrate layout pattern 200 for a first diode and asecond diode in accordance with an embodiment of the present invention.The first diode comprises an array of discrete diode elements d1 and thesecond diode comprises an array of discrete diode elements d2. Separateinterconnects may be fabricated to achieve parallel electricalconnection between diodes d1, and between diodes d2.

In general, the diode arrays d1 and d2 are preferably laid out in anarea with a small area moment (e.g. a square or a circle). A compactlayout on the surface of the integrated circuit minimizes the overallspatially related variations between the diodes. It is also desirablethat each of the diode arrays have a common centroid.

In a preferred embodiment, the exemplary pattern of FIG. 2A comprises128 d1 diodes and 128 d2 diodes laid out in a square with a dimension ofabout 85 microns.

In achieving the desired layout, a sub-array 201 that has a commoncentroid and compact area, may be used as a tile to build the overallarea for the two diodes.

FIG. 2B shows an approximate circular substrate layout pattern 205 thatcomprises the sub-array 201 of FIG. 2A. It is desirable that the totaldiode array have a shape that has two or more axes of symmetry.

FIG. 3A shows a dual-diode thermal sensor 300, in accordance with anembodiment of the present invention. The sensor 300 comprises acontroller 315 for controlling the current output of a first currentsource 305 and a second current source 310. The first current source 305is coupled to a first diode 320, and the second current source 310 iscoupled to a second diode 325. Both diode 320 and diode 325 are coupledto ground.

The output ratio of current source 310 to current source 305 is fixed(I2=K*I1) within the range used for measurement, thereby eliminating thevariable (I₂/I₁) term from the Diode Equation. Each of the currentsources 305 and 310 may comprise an array of current source elements,wherein the arrays share a common centroid.

The voltage drop across each of the diodes 320 and 325 are input to adifferential amplifier that in turn drives an analog-to-digitalconverter (ADC). Since voltage measurements are made simultaneously andthe difference is quantized, only one quantization error is involved inthe measurement.

FIG. 3B shows a dual-diode thermal sensor with a sensing series resistor335, in accordance with an embodiment of the present invention. Theoutput of Controller 315 controls current source 306 and current source311. Current source 306 and current source 311 each comprises M and Nidentical small current source elements (iLSB), respectively. In thisparticular embodiment N is programmable, whereas M is fixed. However, inother embodiments, M may also be programmable.

The controller 315 controls the current level for each of the iLSBcurrent sources that make up sources 306 and 311, and may also controlthe number of iLSB current sources that are active.

Similarly, resistor R1 335 may be made up of a number of identical smallresistors (rLSB). Resistor R1 may be implemented as a programmableresistor by bypass switches (e.g., transistors with low R_(ds) on) forone or more of the rLSB resistors. In this particular embodiment, R1 isconsidered as having a fixed value, with R rLSB resistors in series.

A high-gain op amp (comparator) 340 is coupled to receive V_(A) and V₂as inputs. V_(A) is the sum of the voltage drops across resistor R1 anddiode D1320, and V₂ is the voltage drop across diode D2 325. The outputof the comparator is coupled to a processor 345 that determines thediode temperature from the value of N established by controller 315, anda set of circuit parameters. N may be adjusted by the controller 315until the comparator 340 switches, thus establishing the point at whichV_(A)=V₂.

Since the entire thermometer 301 may fabricated on-chip, the negativeimpact of series resistances may be reduced considerably. The on-chipthermometer may also be tested and adjusted to compensate for observeddeviations in temperature readings.

In testing, the temperature determined by the processor may be comparedto the known value of the integrated circuit during test. A fuse array350 may be used to program a correction that may be read and applied bythe processor. For example an array of four fuses may provide 15 valuesfor correction. In this case, an anticipated error range of +/−3K maythus be divided into 15 corrections that may be applied, ranging from−3K to +3K in increments of 0.4K. More fuses may be used to provide agreater range of corrections and/or a finer resolution of correction.

FIG. 4 shows a current source servo controller 400 that may be used bycontroller 315 for establishing the current level for each of the iLSBsources. An op amp 405 receives a precision reference voltage (e.g.,bandgap reference) as one input. A second input is taken as the voltageacross resistor R2 415, providing a servo loop that operates to set thecurrent through R2 such that the voltage drop across R2 is equal toV_(ref).

In one embodiment, resistor R2 comprises a number of identical smallresistors (rLSB), as are used in R1. In one embodiment, resistor R2 maybe implemented as a programmable resistor by bypass switches (e.g.,transistors with low R_(ds) on) for one or more of the rLSB resistors.In this particular embodiment, R2 is considered as having a fixed value,comprising S rLSB resistors in series.

The servo loop current source Hoop 410 is made up of L iLSB currentsources in parallel, similar to current sources I1 and I2 of FIG. 3B. Inthe example of FIG. 4, op amp (operational amplifier) 405 drives acurrent source 406 that is mirrored by each of the iLSB current sourcesin Iloop, I1 and I2. Alternatively, each iLSB current source may be avoltage controlled current source that is driven by the output of op amp405.

Taking into account the configuration of the controller of FIG. 4 andthe sensor circuit of FIG. 3B, the voltage drop across resistor R1 335may be represented as:

V _(R1) =I1*R1=M*iLSB*R*rLSB=MR*(iLSB*rLSB)

also,

V _(R2) =Iloop*R2=L*iLSB*S*rLSB,

V _(ref) =V _(R2)

Giving:

V _(ref) /LS=iLSB*rLSB

V _(R1)=(MR/LS)*V _(ref)

It is appreciated that the voltage across R1, that is V_(R1), is a knownquantity based upon the integer quantities M, R, L and S, and thereference voltage V_(ref). With this in mind, the operation of thesystem of FIG. 3B is described.

With reference to FIG. 3B, it is seen that since V_(A)=V_(R1)+V₁ andV_(A)=V₂, V_(R1)=V₂−V₁, which is one term required for a temperaturesolution based upon the Diode Equation, with the other term being thecurrent ratio.

Referring again to FIG. 3B, N may be set equal to M by the controller315, causing the voltage V_(A) to be greater than V₂. The programmablecurrent source I2 311 may then be incrementally adjusted by sequentiallyswitching in additional iLSB current sources in turn. Alternatively, abinomial search or other algorithm may be used to find the value of N atwhich V_(A)=V₂.

At some point, when a number J of additional iLSB sources have beenswitched in, V₂ will exceed V_(A), causing the comparator 340 to changestate. Although the incremental nature of the current increases preventsdetermining the exact current at which V_(A)=V₂, a range can beestablished and the range midpoint used for purposes of calculation. Inthis case, the current may be taken as (M+J−½)iLSB.

Thus, the current ratio I₂/I₁ corresponding to the diode voltages V₁ andV₂ has been determined as (M+J−½)/M. Although a specific scheme has beenpresented for adjusting the current source I2 and equalizing V_(A) andV₂, other starting values and modes of adjustment may be used. Also,another value within the comparator crossover range may used forpurposes of calculation.

There are many factors to be considered in the selection of thecomponent sizes and the values of the integers L, M, R, and S. Dependingupon the process used for integrated circuit fabrication and the designof the circuit, the transistors (switches and current sources), diodesand resistors of the thermal sensor may vary considerably in size andnumber.

It is desirable to provide a common centroid for each of the currentsources Iloop, I1, and I2. The centroid applies to the layout of thearrays of iLSB sources that make up each of the current sources Iloop,I1, I2. The centroid also applies to the subset of the current sourcesthat may be switched on at a particular time. Thus, there is both acentroid associated with layout, and a centroid associated withoperation.

For example, in a circuit for which the diode current limit was desiredto be about 400 microamperes, 256 iLSB current sources with a nominalcurrent of roughly 2 to 2.5 microamperes may be used. The maximumcurrent is related to the minimum temperature that is to be measuredaccurately. Since high temperature accuracy is generally more importantfor a circuit than low temperature accuracy, the minimum accuratetemperature is selected to be about 308K to 318K. It is desired to use asufficiently high value for N (e.g., 120 to 160) at temperatures ofinterest (e.g., 340K to 385K) in order to minimize the impact ofquantization error.

The error in the temperature measurement will be proportional to theerror in the (V₂−V₁). Therefore, it is desirable to make (V₂−V₁) aslarge as possible relative to the resolution of the comparator. On theother hand, I₂ and V₂ should be kept from being too large and I₁ and V₁should be kept from being to small. In this way, deviation from theDiode Equation can be minimized, and the ideality factor n kept closeto 1. In view of these considerations dV=(V₂−V₁) may be targeted to beabout 0.085 volts.

It is desired that the current ratio N/M be about 18 for the lowesttemperature of interest: N/M=exp((q*dV)/(k*T_(min))), where N=160,corresponding to the T_(min) of interest. Using dV=0.085 V,T_(min)=340K, we get N/M=18.2, M=160/18.2=8.8, giving 9 for the fixedinteger value of M.

The V_(ref) level may be obtained from a bandgap reference. In order toreduce noise in reference signal, a voltage divider using two high valueresistors may be used to divide the bandgap voltage. For example, twomatched resistors may be used to divide a bandgap voltage of 1.175 voltsin half to provide a V_(ref)=0.5875 volts. The divider output may use ashunt capacitor to filter high frequency noise. The sensor circuit servomay be used to filter low frequency noise.

The relationships for S, L and R may now be examined. From above,SL=MR*(V_(ref)/dV)=R1*62.2. To avoid having to make S and L too large,it is preferred that R be set close to 1. Using S=L=8, we get evennumbers that help the centroiding for the resistors.

The calculated dV is then: dV=(MR/SL)*vRef=0.0826 V. Finally, a nominalvalue for rLSB is determined from rLSB=dV/(M*iLSB); using a nominalvalue of 2.0 microamperes for iLSB, rLSB=4.6 kohm. Thus, the solutionfor T from the Diode Equation becomes:

$\begin{matrix}{T = {\left( {q/{nk}} \right)*{{dV}/{\ln \left( {i\; {2/i}\; 1} \right)}}}} \\{= {\left( {q/{nk}} \right)*\left( {\left( {{MR}*{V_{ref}/{SL}}} \right)/{\ln \left( {N/M} \right)}} \right)}} \\{= {11600K\text{/}V*{\left( {9*{\left( {1.175\mspace{14mu} V\text{/}2} \right)/64}} \right)/{{\ln \left( {N/9} \right)}.}}}} \\{{958.4/{\ln \left( {N/9} \right)}}}\end{matrix}$

TABLE 1 TEMPERATURE N*iLSB (K) N (microamperes) 398.0 100 218 396.4 101220 370.0 120 258 368.8 121 260 333.7 159 336 333.0 160 338 309.0 200418 308.5 201 420

Table 1 shows the calculated temperature for different values of N inaccordance with the above solution for T. The step from N=100 to N=101corresponds to a change in temperature from 398K to 396.4K, or adifference of 1.6K. In keeping with the practice of taking the midpointof the interval in which the comparator changes state, the temperaturemeasurement for this interval would be 397.2K, with a quantization errorof +1-0.8K.

Although the quantization error for the interval between N=200 and N=201is smaller than that for the interval for N=100 to N=101, it should benoted that the diode current is over 400 microamperes, and ohmic effectsmay affect the overall accuracy.

Although the simultaneous use of two diodes obviates the need forsequential measurements, the system of the present invention may be usedto make sequential measurements using two different current levels.Thus, temperature measurements may be made in a conventional timeframe,but with increased accuracy.

In general, the accuracy requirement for temperature measurements onintegrated circuits is on the order of +1-3K. Although the quantizationmay be reduced by using a 9-bit digital-to-analog converter (DAC)instead of the 8-bit DAC (256 iLSB sources) described herein, thequantization error of 0.8K is already low with respect to industrystandards for overall accuracy.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications are suitedto the particular use contemplated. It is intended that the scope of theinvention be defined by the Claims appended hereto and theirequivalents.

1. A controller for an on-chip thermometer comprising: an operationalamplifier having a first input coupled to a voltage reference, a secondinput and an output for coupling to said thermometer; a current sourcecoupled to said output of said operational amplifier and to first andsecond diodes, wherein said current source comprises an array of currentsource elements; and a resistor having a first terminal and a secondterminal, wherein said first terminal is coupled to said current sourceand to said second input of said operational amplifier; wherein saidcontroller is operable to cause a first current in said first diode tobe fixed while varying a second current in said second diode responsiveto a feedback signal to alter the ratio of said first and secondcurrents.
 2. A current source servo circuit for establishing the currentlevel for current sources used in a system for measuring temperature inan integrated circuit, said current source servo circuit comprising: asingle output coupled to a fixed current source and also a variablecurrent source; and a voltage source coupled to an input of anoperational amplifier; and a voltage across a resistor coupled to theother input of said operational amplifier.
 3. A current source servocircuit according to claim 2 wherein said voltage source is a band gapreference.
 4. A current source servo circuit according to claim 2wherein said resistor is related by construction to a resistor in saidsystem for measuring temperature in an integrated circuit.
 5. A currentsource servo circuit according to claim 5 and further for establishing aknown voltage in said system for measuring temperature in an integratedcircuit.
 6. A servo circuit comprising: an output coupled to first andsecond thermal sensing diodes that are coupled to a comparator; and aninput coupled to said comparator and receiving a feedback signaltherefrom; wherein said servo circuit is operable to: cause a firstfixed current in said first diode to establish a first voltage at afirst input of said comparator while varying a second current throughsaid second diode to establish a second voltage responsive to saidfeedback signal; cause said first and second voltages to be equalresponsive to said feedback signal; and
 7. A servo circuit as recited inclaim 6, wherein said servo circuit is further operable to establish aknown voltage, wherein said first voltage comprises said known voltageand a voltage across said first diode.